FIG. 9 is a diagram showing an example of a conventional backlight unit for illuminating a liquid crystal display panel.
In FIG. 9, light 62 and 64 emitted from a light source 14 made up of LEDs (Light-Emitting Diodes), for example, advances through a lightguide plate 12 while repeating reflection therein. While advancing, some light 62 is reflected to exit the upper surface of the lightguide plate 12 by grooves provided on the bottom of the lightguide plate 12. The light 62 exiting the lightguide plate 12 is diffused by a diffuser sheet 38 and then passed through prism sheets 40 and 42 so as to be incident perpendicularly on a liquid crystal display panel 44.
Some light 64 exits the lower surface of the lightguide plate 12, but it is reflected by a reflective sheet 60 back into the lightguide plate 12. Thereafter, the light 64 exits the upper surface of the lightguide plate 12 to illuminate the liquid crystal display panel 44 in the same way as the light 62.
The diffuser sheet 38 is used to diffuse light so as to illuminate the liquid crystal display panel 44 uniformly. It is required in order to illuminate the liquid crystal display panel 44 even more uniformly that the lightguide plate 12 should also emit light uniformly over the whole light-emitting surface thereof.
Japanese Patent Application Publication No. 2004-69879 proposes a technique wherein a light-emitting surface of a lightguide plate is provided with ridges having a triangular section such that the pitch of the ridges decreases with ridges being situated farther away from the associated light source increases, thereby making the brightness uniform over the entire light-emitting surface of the lightguide plate.
The proposed technique is, however, for a unit adapted to illuminate only a liquid crystal display device provided at the front side of a lightguide plate. It is not suitable for a unit that illuminates liquid crystal display devices provided at both the front and back sides of a lightguide plate.
Japanese Patent Application Publication No. 2004-144990 discloses a double-sided emitting backlight unit that illuminates both a large first liquid crystal display panel provided on the front side of the backlight unit and a small second liquid crystal display panel provided on the back side thereof. In this backlight unit, a semitransmitting reflector is provided on the back side thereof, and an opening is provided in a portion of the semitransmitting reflector corresponding to the second liquid crystal display panel to pass light from a lightguide plate to the second liquid crystal display panel. The rest of the semitransmitting reflector transmits part of light and reflects the remaining light toward the first liquid crystal display panel.
With this backlight unit, therefore, light transmitted through the part of the semitransmitting reflector other than the opening is wasted, resulting in a degradation of the backlight utilization efficiency. If a reflecting member is provided over an area where the second liquid crystal display panel is not present to reflect substantially all light directed to this area back toward the first liquid crystal display panel with a view to effectively utilizing light, the intensity of illuminating light becomes lower in a region of the first liquid crystal display panel that corresponds to the second liquid crystal display panel than in the rest thereof, causing luminance unevenness on the first liquid crystal display panel.